This invention relates to a pulse burner incorporated into a water heater or air heater utilized in residences, shops or offices of relatively small size.
In prior art combustion systems, such continuous combustion systems as Bunzen type or Gun type have been used in which gas (usually town gas or propane gas) is burnt continuously with a burner disposed at the lower portion, and the combustion gas is sent to a heat exchanger to heat water or air. In this system, in order to increase the heat efficiency it is necessary to increase the heat transfer area of the heat exchanger or to increase the number or density of combustion gas flow passages. This not only increases the sizes of the heater and fan supplying combustion air but also complicates the mechanism.
In recent years, so called pulse burners have been developed and are disclosed in many publications. In principle, the pulse burner resembles a two cycle gasoline engine, in which fuel and air are sucked into a combustion chamber of a relatively small volume, the mixture is caused to explode by an ignition plug and then exhausted through a heat exchanger in the form of a relatively fine pipe. When explosion occurs, the pressure in the combustion chamber increases greatly thereby to close a flapper valve for stopping suction of the air-fuel mixture. Under this condition, the combustion gas is exhausted to the outside of a building via the heat exchanger, thereby creating a negative pressure therein. Then the flapper valve opens automatically to suck again the air-fuel mixture into the combustion chamber. This cycle of operation is repeated at a frequency of 35-80 Hz/sec. The pulse burner has revolutionary advantages over conventional gas or oil burners. More particularly, its heating efficiency is 91 to 96%, meaning saving of fuel and miniaturization of the apparatus, and it does not need a chimney because the combustion product is exhausted at a high speed and high pressure and because the temperature of the exhaust gas is lower than 38.degree. C. Accordingly, an inexpensive plastic pipe (polyvinyl chloride pipe, for example) can be used to extend horizontally to the outdoor through the wall of a building. Without a vertical chimney, heat loss during off cycle is very small. Unexpectedly, the noise cause by frequent explosions is less than 100 dB and can be reduced substantially by suitable means. Moreover, the concentration of the harmful components of the exhaust gas, for example CO, is of the order of less than 0.025% and can be reduced to 0.01-0.005% by volume by suitably selecting the volume of the combustion chamber and the diameter of the exhaust pipe.
A flame detector is installed in the combustion chamber to detect failure of ignition, i.e., explosive combustion of the air-fuel mixture in which case supply of fuel gas and the operation of the ignition plug are stopped and not exploded air-gas mixture in the combustion chamber is purged outside thereof by admitting clean air. Then, the fuel gas is admitted to form a fresh air-gas mixture and the ignition plug is energized to restart the operation. According to this invention, the flame detector detects combustion or flame by sensing electric current flowing through the flame. Such flame detector can be used to control the ignition plug circuit, an electromagnetic valve in a fuel supply conduit and a motor operated fan supplying air into the combustion chamber as disclosed in a copending application filed on the same day and assigned to the same assignee.
Heretofore, combustion or flame in the combustion chamber has been detected by a thermocouple attached to the side wall of the combustion chamber. Where a thermocouple is used to measure the temperature of the combustion chamber or exhaust gas, it often misoperates, so that fresh fuel gas, that is not yet burnt fuel gas is exhausted during pulse combustion because the temperature of the side wall and hence of the thermostat does not decrease at once even when an ignition miss occurs during normal operation.